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<h1>Commands</h1>
<p>
Commands such as <b>Delete</b> or <b>Paste</b> or <b>Undo</b> are implemented by the <a>CommandHandler</a> class.
</p>
<p>
Keyboard events, like mouse and touch events, always go to the <a>Diagram.currentTool</a>.
The current tool, when the user is not performing some gesture, is the same as the <a>Diagram.defaultTool</a>,
which normally is the <a>Diagram.toolManager</a>.
The <a>ToolManager</a> handles keyboard events by delegating them to the <a>Diagram.commandHandler</a>.
</p>
<p>
Basically, the diagram handles a keyboard event, creates an <a>InputEvent</a> describing it,
and then calls <a>ToolManager.doKeyDown</a>.  That in turn just calls <a>CommandHandler.doKeyDown</a>.
The same sequence happens for key-up events.
</p>
<p>
Please note that the handling of keyboard commands depends on the diagram getting focus and then getting keyboard events.
Do not apply any styling such as <pre class="lang-css"><code>canvas:focus { display: none; }</code></pre>.
</p>

<h2 id="KeyboardCommandBindings">Keyboard command bindings</h2>
<p>
The <a>CommandHandler</a> implements the following command bindings for keyboard input:
</p>
<ul>
  <li>Del &amp; Backspace invoke <a>CommandHandler.deleteSelection</a></li>
  <li>Ctrl-X &amp; Shift-Del invoke <a>CommandHandler.cutSelection</a></li>
  <li>Ctrl-C &amp; Ctrl-Insert invoke <a>CommandHandler.copySelection</a></li>
  <li>Ctrl-V &amp; Shift-Insert invoke <a>CommandHandler.pasteSelection</a></li>
  <li>Ctrl-A invokes <a>CommandHandler.selectAll</a></li>
  <li>Ctrl-Z &amp; Alt-Backspace invoke <a>CommandHandler.undo</a></li>
  <li>Ctrl-Y &amp; Alt-Shift-Backspace invoke <a>CommandHandler.redo</a></li>
  <li>Up &amp; Down &amp; Left &amp; Right (arrow keys) call <a>Diagram.scroll</a></li>
  <li>PageUp &amp; PageDown call <a>Diagram.scroll</a></li>
  <li>Home &amp; End call <a>Diagram.scroll</a></li>
  <li>Space invokes <a>CommandHandler.scrollToPart</a></li>
  <li>Keypad-- (minus) invokes <a>CommandHandler.decreaseZoom</a></li>
  <li>Keypad-+ (plus) invokes <a>CommandHandler.increaseZoom</a></li>
  <li>Ctrl-0 invokes <a>CommandHandler.resetZoom</a></li>
  <li>Shift-Z invokes <a>CommandHandler.zoomToFit</a>; repeat to return to the original scale and position</li>
  <li>Ctrl-G invokes <a>CommandHandler.groupSelection</a></li>
  <li>Ctrl-Shift-G invokes <a>CommandHandler.ungroupSelection</a></li>
  <li>F2 invokes <a>CommandHandler.editTextBlock</a></li>
  <li>Menu Key invokes <a>CommandHandler.showContextMenu</a></li>
  <li>Esc invokes <a>CommandHandler.stopCommand</a></li>
</ul>
<p>
On a Mac the Command key is used as the modifier instead of the Control key.
</p>
<p>
At the current time there are no keyboard bindings for commands such as <a>CommandHandler.collapseSubGraph</a>,
<a>CommandHandler.collapseTree</a>, <a>CommandHandler.expandSubGraph</a>, or <a>CommandHandler.expandTree</a>.
</p>
<p>
If you want to have a different behavior for the arrow keys, consider using the sample class extended from <a>CommandHandler</a>:
<a href="../extensions/DrawCommandHandler.js">DrawCommandHandler</a>, which implements options for having
the arrow keys move the selection or change the selection.
</p>
<p>
That DrawCommandHandler extension also demonstrates a customization of the <b>Copy</b> and <b>Paste</b> commands
to automatically shift the location of pasted copies.
</p>

<h2 id="CommandHandler">CommandHandler</h2>
<p>
The <a>CommandHandler</a> class implements pairs of methods:
a method to execute a command and a predicate that is true when the command may be executed.
For example, for the <b>Copy</b> command, there is a <a>CommandHandler.copySelection</a> method
and a <a>CommandHandler.canCopySelection</a> method.
</p>
<p>
Keyboard event handling always calls the "can..." predicate first.
Only if that returns true does it actually call the method to execute the command.
</p>
<p>
There are a number of properties that you can set to affect the CommandHandler's standard behavior.
For example, if you want to allow the user to group selected parts together with the <a>CommandHandler.groupSelection</a>,
you will need to set <a>CommandHandler.archetypeGroupData</a> to a group node data object:
</p>
<pre class="lang-js"><code>
  diagram.commandHandler.archetypeGroupData =
    { key: "Group", isGroup: true, color: "blue" };
</code></pre>
<p>
That data object is copied and added to the model as the new group data object by <a>CommandHandler.groupSelection</a>.
</p>
<p>
If you want to add your own keyboard bindings, you can override the <a>CommandHandler.doKeyDown</a> method.
For example, to support using the "T" key to collapse or expand the currently selected <a>Group</a>:
</p>
<pre class="lang-js"><code>
    myDiagram.commandHandler.doKeyDown = function() {
      var e = myDiagram.lastInput;
      var cmd = myDiagram.commandHandler;
      if (e.key === "T") {  // could also check for e.control or e.shift
        if (cmd.canCollapseSubGraph()) {
          cmd.collapseSubGraph();
        } else if (cmd.canExpandSubGraph()) {
          cmd.expandSubGraph();
        }
      } else {
        // call base method with no arguments
        go.CommandHandler.prototype.doKeyDown.call(cmd);
      }
    };
</code></pre>
<p>
Do not forget to call the base method in order to handle all of the keys that your method does not handle.
</p>
<p class="box bg-info">
Note that calling the base method involves getting the base class's prototype's method.
If the base method takes arguments, be sure to pass arguments to the call to the base method.
</p>

<h2 id="UpdatingCommandUI">Updating command UI</h2>
<p>
It is common to have HTML elements outside of the diagram that invoke commands.
You can use the <a>CommandHandler</a>'s "can..." predicates to enable or disable UI that would invoke the command.
</p>
<pre class="lang-js" id="commands"><code>
  // allow the group command to execute
  diagram.commandHandler.archetypeGroupData =
    { key: "Group", isGroup: true, color: "blue" };
  // modify the default group template to allow ungrouping
  diagram.groupTemplate.ungroupable = true;

  var nodeDataArray = [
    { key: "Alpha" },
    { key: "Beta" },
    { key: "Delta", group: "Epsilon" },
    { key: "Gamma", group: "Epsilon" },
    { key: "Epsilon", isGroup: true }
  ];
  var linkDataArray = [
    { from: "Alpha", to: "Beta" },
    { from: "Beta", to: "Beta" },
    { from: "Gamma", to: "Delta" },
    { from: "Delta", to: "Alpha" }
  ];
  diagram.model = new go.GraphLinksModel(nodeDataArray, linkDataArray);

  // enable or disable a particular button
  function enable(name, ok) {
    var button = document.getElementById(name);
    if (button) button.disabled = !ok;
  }
  // enable or disable all command buttons
  function enableAll() {
    var cmdhnd = diagram.commandHandler;
    enable("SelectAll", cmdhnd.canSelectAll());
    enable("Cut", cmdhnd.canCutSelection());
    enable("Copy", cmdhnd.canCopySelection());
    enable("Paste", cmdhnd.canPasteSelection());
    enable("Delete", cmdhnd.canDeleteSelection());
    enable("Group", cmdhnd.canGroupSelection());
    enable("Ungroup", cmdhnd.canUngroupSelection());
    enable("Undo", cmdhnd.canUndo());
    enable("Redo", cmdhnd.canRedo());
  }
  // notice whenever the selection may have changed
  diagram.addDiagramListener("ChangedSelection", function(e) {
    enableAll();
  });
  // notice when the Paste command may need to be reenabled
  diagram.addDiagramListener("ClipboardChanged", function(e) {
    enableAll();
  });
  // notice whenever a transaction or undo/redo has occurred
  diagram.addModelChangedListener(function(e) {
    if (e.isTransactionFinished) enableAll();
  });
  // perform initial enablements after everything has settled down
  setTimeout(enableAll, 100);
  // make the diagram accessible to button onclick handlers
  myDiagram = diagram;
  // calls enableAll() due to Model Changed listener
  myDiagram.undoManager.isEnabled = true;
</code></pre>
<script>goCode("commands", 600, 150)</script>
<input id="SelectAll" type="button" onclick="myDiagram.commandHandler.selectAll()" value="Select All" />
<input id="Cut" type="button" onclick="myDiagram.commandHandler.cutSelection()" value="Cut" />
<input id="Copy" type="button" onclick="myDiagram.commandHandler.copySelection()" value="Copy" />
<input id="Paste" type="button" onclick="myDiagram.commandHandler.pasteSelection()" value="Paste" />
<input id="Delete" type="button" onclick="myDiagram.commandHandler.deleteSelection()" value="Delete" />
<input id="Group" type="button" onclick="myDiagram.commandHandler.groupSelection()" value="Group" />
<input id="Ungroup" type="button" onclick="myDiagram.commandHandler.ungroupSelection()" value="Ungroup" />
<input id="Undo" type="button" onclick="myDiagram.commandHandler.undo()" value="Undo" />
<input id="Redo" type="button" onclick="myDiagram.commandHandler.redo()" value="Redo" />

<p>
Each button is implemented in the following fashion:
</p>
<pre class="lang-html"><code>
&lt;input id="SelectAll" type="button"
       onclick="myDiagram.commandHandler.selectAll()" value="Select All" /&gt;
</code></pre>
<p>
Whenever the selection changes or whenever a transaction or undo or redo occurs,
the enableAll function is called to update the "disabled" property of each of the buttons.
</p>

<h2 id="Accessibility">Accessibility</h2>
<p>
Since <b>GoJS</b> is based on the HTML Canvas element,
making an app that is accessible to screen-readers or other accessibility devices
is a matter of generating fallback content outside of GoJS,
just as you would generate fallback content separate from any HTML Canvas application.
</p>
<p>
Although much of the predefined functionality of the <a>CommandHandler</a> is accessible
with keyboard commands or the default context menu, not all of it is,
and the functionality of the <a>Tool</a>s mostly depends on mouse or touch events.
We recommend that you implement alternative mechanisms specific to your application
for those tools that you want your users to access without a pointing device.
</p>

<h2 id="MoreCommandHandlerOverrideExamples">More CommandHandler override examples</h2>

<p>
Stop CTRL+Z/CTRL+Y from doing an undo/redo, but still allow <a>CommandHandler.undo</a> and <a>CommandHandler.redo</a> to be called programatically:
</p>
<pre class="lang-js"><code>
  myDiagram.commandHandler.doKeyDown = function() {
    var e = myDiagram.lastInput;
    // The meta (Command) key substitutes for "control" for Mac commands
    var control = e.control || e.meta;
    var key = e.key;
    // Quit on any undo/redo key combination:
    if (control && (key === 'Z' || key === 'Y')) return;

    // call base method with no arguments (default functionality)
    go.CommandHandler.prototype.doKeyDown.call(this);
  };
</code></pre>

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